The invention pertains to an optical waveguide and a fiberoptic isolator containing the waveguide.
Fiberoptic components and sensors are gaining increasing importance in the transmission and processing of signals in optical communications systems and in many fiberoptic devices/systems. Fiberoptic devices/systems ordinarily contain at least one light-transmitting optical fiber (optical waveguide, glass fiber), a signal-processing component and/or a sensor, as well as a source (laser or laser diode) emitting coherent radiation.
In the transmission of signals over very long paths, such as intercontinental transmission, it is necessary to amplify the signal at regular intervals.
In most Fiberoptic systems it must be assured that optical signals are not back-scattered into the laser light source or the optical amplifier, since this may bring about undesired oscillations. Moreover, the back-scattered light increases the noise level of the system.
To solve this problem, isolators are installed in fiberoptic systems and optical amplifiers. They guarantee that light is transmitted in only one direction, and that propagation in the opposite direction is largely suppressed.
A commonly used optical isolator is the so-called xe2x80x9cbulkxe2x80x9d isolator. Here, a magnetooptical crystal subjected to an external magnetic field is arranged between two polarizers whose directions of polarization enclose an angle of 45xc2x0. Due to the magnetooptical effect (Faraday effect), the plane of polarization of the incident light is rotated by 45xc2x0, independently of its initial orientation. The incident linearly polarized light thus passes through the first polarizer rotating the plane of polarization by 45xc2x0, so that it can pass through the second polarizer unattenuated. The plane of polarization of the back-scattered light reaching the second polarizer is likewise rotated by 45xc2x0, but is thus displaced by 90xc2x0 with respect to the polarization direction of the first polarizer and cannot pass through it.
The use of a magnetooptical film in place of a magnetooptical crystal is also known.
Along with these xe2x80x9cbulkxe2x80x9d isolators, so called xe2x80x9call-fiberxe2x80x9d insulators are also used (see, for instance, U.S. Pat. No. 5,479,542). Although the magnetooptical effect in the glass fiber is exploited in this type of isolator, an additional device for generating an external nagnetic field is necessary. This has the disadvantage that the optical components are comparatively large and cannot be built into the cable. Additionally, the aforementioned isolators are extremely temperature- and humidity-sensitive. They must therefore be protected from environmental influences and arranged, for example, in a closed container such as a sleeve. For certain network infrastructure such as oceanic cable or aerial cable networks, this is not possible at all or is possible only at great expense.
The problem to be solved by the invention is therefore to create an optical waveguide serving as a polarization rotator and which can be integrated into an optical waveguide system.
Another problem to be solved by the invention is to provide a fiberoptic isolator that avoids the above-mentioned disadvantages.
According to the invention, problems mentioned above are solved by an optical waveguide according to claim 1 and by an optical isolator according to claim 5. The subordinate claims pertain to additional advantageous aspects of the invention.
The optical waveguide according to the invention contains a core whose material has a sufficiently large Faraday effect, as well as a magnetic or magnetizable outer coating that generates a permanent magnetic field producing the Faraday effect. Such a waveguide can be integrated into ordinary waveguide systems and easily joined to other waveguides (glass fibers, LWL cores, LWL fiber tapes and so on), in particular, by splicing.
In accordance with one aspect of the invention, the outer coating is formed by two half-shells, one half-shell constituting the magnetic south pole and the other the magnetic north pole. The magnetic field generated by the half-shells can be relatively weak, as long as the effective length, that is, the length of the half-shells enclosing the fiber core is selected to be sufficiently large, for instance, 10 m.
It has proven especially advantageous to dope the fiber core, normally consisting of quartz glass, with YIG material, which exhibits a sufficiently large Faraday effect.
Preferably, the optical waveguide according to the invention is used as a single waveguide.
The optical isolator according to the invention is a fiberoptic isolator with at least one polarizer and one polarization rotator with an optical waveguide that has a core having a sufficiently large Faraday effect and an outer coating. According to the invention, the outer coating is configured such that it generates a permanent magnetic field in the core.
According to another advantageous aspect of the invention, the polarizer comprises a polarization-maintaining or a polarization-rotating glass fiber, where the fibers of the polarizer and the polarization rotator are constructed in one piece as spliced, continuous optical glass fibers.